1. Field of the Invention
The present invention relates to a feedback gain adjusting method and a feedback gain adjusting device of a tracking-type laser interferometer and a tracking-type laser interferometer. Particularly, the present invention relates to a feedback gain adjusting method and a feedback gain adjusting device of a tracking-type laser interferometer and a tracking-type laser interferometer, which can stabilize an optical axis tracking control system of the tracking-type laser interferometer by optimizing feedback gain of tracking control without using an absolute distance between a retro reflector and the laser interferometer.
2. Description of Related Art
A tracking-type laser interferometer is known which is configured by a laser interferometer mounted with an position sensitive detector detecting an offset amount of an optical axis of return light, a biaxial rotation mechanism turning the laser interferometer toward an arbitrary direction, and a retro reflector such as a retro reflector fixated to a measured object (see Japanese Patent Laid-open Publication No. S63-231286 and Japanese Unexamined Patent Publication No. 2007-057522 ). In this example, the retro reflector is an optical element in which reflected light returns parallel to incident light (also referred to as measurement light). The incident light becomes coaxial with emitted light when reflected at a center of the retro reflector. Therefore, based on an output of the position sensitive detector, an interference length measurement is possible in the arbitrary direction by controlling the biaxial rotation mechanism such that the offset amount of the optical axis is 0.
As shown in FIG. 1, a common tracking-type laser interferometer is configured to include a laser interferometer 101 (referred to simply as an interferometer), a measurement head 103 having an position sensitive detector 102 installed which detects the offset amount between the optical axes of the measurement light and the reflected light, a biaxial rotation mechanism 104 turning the measurement head 103 to the arbitrary direction, an angle sensor 105 detecting a rotation angle of the biaxial rotation mechanism 104, a retro reflector 107 fixated to a measured object 106, and a controller 108 which performs tracking of the measured object 106 and measurement data collection.
The controller 108 collects a distance signal from the interferometer 101, the optical axis offset amount from the position sensitive detector 102, and an angle signal from the biaxial rotation mechanism 104 to drive the biaxial rotation mechanism 104 so as to keep the optical axis offset amount at 0.
When the retro reflector 107 is displaced, the reflected light (referred to as return light) returning to the interferometer 101 is shifted to be parallel to the measurement light and enters the interferometer 101. By using the position sensitive detector 102 detecting the offset of the optical axis at this point and the biaxial rotation mechanism 104, feedback control is performed such that the optical axis of the measurement light travels to the center of the retro reflector 107 and tracking control is performed.
While the rotation angle is required to control the biaxial rotation mechanism 104, the feedback control is performed using the optical axis offset, and therefore, gain of a control system may become excessive or insufficient based on an absolute distance between the retro reflector 107 and the laser interferometer 101.
Accordingly, the absolute distance between the retro reflector and the laser interferometer needs to be entered using some method and an inventor suggests an absolute distance estimation method in Japanese Patent Laid-open Publication No. 2009-229066.
However, until the absolute distance is entered, the optical axis tracking control system of the tracking-type laser interferometer is unstable.